1. Field of the Invention
This invention relates to signal transmission ranging systems such as radar, sonar, positioning systems and the like. The new method provides a means to reduce system errors caused by reception of secondary path signals from reflecting surfaces.
2. Description of Related Art
Radio ranging systems use the time of propagation of radio signals to obtain the range (distance) between points of interest. A now classic example of this is radar. With radar the observed time of the transmitted radar signal in propagating to the target and returning to the radar receiver, then scaled by the speed of propagation, accomplishes ranging. Sonar is a direct acoustic analog of radar and is included as a ranging system also amenable to application of the subject invention. With the Global Positioning System, GPS, or GPS-like satellite positioning systems the time at which a satellite signal is received is compared to the time the satellite transmitted that signal, information available to the user, and then scaled by propagation speed to obtain range from satellite to receiver. In satellite positioning systems user's position is the information generally desired and is obtained by exploiting understood trilateration techniques using the inferred ranges to a constellation of satellites whose positions in space are a priori known.
In radio and acoustic ranging systems a commonly observed effect occurs due to the reception of one or more secondary path signals associated with reflecting surfaces positioned to provide those signals. In television, multipath, the term used to denote this phenomenon, manifests as undesired echoes or ghosts of the image transmitted. In radio ranging systems multipath manifests as errors in range which cannot be directly compensated unless the parameters of the multipath signal are known. Although, the effect of multipath signals is generally deleterious in radar and sonar the information on the multipath signals has been used to assist in defining target position. To illustrate the severity of multipath effects in an exemplary radio ranging system, GPS, the ranging error incurred due to multipath, with and without the use of a current state-of-the-art technique to reduce those effects, is shown in FIG. 1.
Multipath interference is coherent, or nearly so, with respect to the direct path signal, i.e., it has the same or very nearly the same spectrum as the direct path signal. For that reason it is particularly difficult to mitigate its effects; filtering and other such commonly used means in the art of suppressing undesired signals are ineffective with multipath.
A number of methods have been put to use with modest success to mitigate multipath effects in radar and GPS. The simplest is the use of a signal receiving antenna that substantially reduces the response of the receiving apparatus to wavefronts originating from the presumed direction of the reflected signal. When the direct and reflected signals arrive at the antenna from the same or nearly the same direction there is experienced an undesired degradation of the receiving apparatus response to the direct path signal, an effect which can seriously compromise ranging performance.
More significant for mitigating multipath effects are methods broadly described as algorithmic. One of the known multipath mitigation algorithms now in use demonstrates a performance described by the algorithm designers shown in FIG. 1. As described in the literature, the technique employs methods for decomposing the signal correlation function to permit inferring the multipath parameters: wavefront intensity, path separation delay, and path phase shift, of the primary multipath wavefront relative to the direct path wavefront, and thereby backing out or removing its effects on the measurement of the direct path delay. This and other such algorithms are now implemented in receivers which incorporate microprocessors or special purpose computing apparatus to execute code embodying these algorithms.
The present invention method or process to multipath mitigation in GPS or radar or sonar receivers or other similarly intended ranging systems implements a substantially different method of operation than cited above and provides an improved level of performance, in fact, near, or at, the limiting bounds of performance and with a substantially reduced computational burden. This is a crucial result because it makes feasible in practice obtaining accuracies of position fixes not heretofore achievable, thereby allowing numerous applications in the fields of radar, sonar and GPS or GPS-like satellite positioning systems not otherwise feasible. In sum, the accuracy achieved with the subject invention exceeds the accuracy achieved by the current art by a substantial factor. The computational efficiency with the invention permits achieving this level of performance with a parsimony of computational apparatus not achieved in the present art.